A single misunderstanding turned longtime friends into enemies.
Tips
history
Found in 《红楼梦》 Chapter 57: '怜新弃旧,反目成仇的多着呢' — 'plenty of people fall in love anew, cast off the old, and turn into enemies.' The phrase was already common in Qing vernacular.
memory
反目 literally 'eyes turned against' — from 《周易》 imagery of eyes no longer meeting. 成仇 = 'become enemies.' The sequence: friendship/family → turn eyes against each other → become enemies.